Low-bandgap, monolithic, multi-bandgap, optoelectronic devices

ABSTRACT

Low bandgap, monolithic, multi-bandgap, optoelectronic devices ( 10 ), including PV converters, photodetectors, and LED&#39;s, have lattice-matched (LM), double-heterostructure (DH), low-bandgap GaInAs(P) subcells ( 22, 24 ) including those that are lattice-mismatched (LMM) to InP, grown on an InP substrate ( 26 ) by use of at least one graded lattice constant transition layer ( 20 ) of InAsP positioned somewhere between the InP substrate ( 26 ) and the LMM subcell(s) ( 22, 24 ). These devices are monofacial ( 10 ) or bifacial ( 80 ) and include monolithic, integrated, modules (MIMs) ( 190 ) with a plurality of voltage-matched subcell circuits ( 262, 264, 266, 270, 272 ) as well as other variations and embodiments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/285,468, filed May 22, 2014, which is a continuation of U.S.application Ser. No. 13/664,142, filed Oct. 30, 2012, which is adivisional of U.S. application Ser. No. 10/515,243, which is a NationalStage entry of PCT/US02/16101, filed May 21, 2002, the disclosures ofwhich are hereby incorporated by reference in their entireties.

GOVERNMENT LICENSE RIGHTS

The United States Government has rights in this invention under ContractNo. DE-AC36-99GO10337 between the United States Department of Energy andthe National Renewable Energy Laboratory, a Division of the MidwestResearch Institute.

TECHNICAL FIELD

This application relates to optoelectronic devices, and, morespecifically, to low bandgap, monolithic, multi-bandgap solarphotovoltaic (SPV) and thermophotovoltaic (TPV) cells for convertingsolar and/or thermal energy to electricity as well as for relatedphotodetector devices for detecting light signals and light emittingdiode (LED) devices for converting electricity to light and/or infrared(IR) radiant energy.

BACKGROUND

It is well known that the most efficient conversion of radiant energy toelectrical energy with the least thermalization loss in semiconductormaterials is accomplished by matching the photon energy of the incidentradiation to the amount of energy needed to excite electrons in thesemiconductor material to transcend the bandgap from the valence band tothe conduction band. However, since solar radiation and blackbodyradiation usually comprise a wide range of wavelengths, use of only onesemiconductor material with one bandgap to absorb such radiant energyand convert it to electrical energy will result in large inefficienciesand energy losses to unwanted heat.

Ideally, there would be a semiconductor material with a bandgap to matchthe photon energy for every wavelength in the radiation. That kind ofdevice is impractical, if not impossible, but persons skilled in the artare building monolithic stacks of different semiconductor materials intodevices commonly called tandem converters and/or monolithic,multi-bandgap or multi-bandgap converters, to get two, three, four, ormore bandgaps to match more closely to different wavelengths ofradiation and, thereby, achieve more efficient conversion of radiantenergy to electrical energy. Essentially, the radiation is directedfirst into a high bandgap semiconductor material, which absorbs theshorter wavelength, higher energy portions of the incident radiation andwhich is substantially transparent to longer wavelength, lower energy,portions of the incident radiation. Therefore, the higher energyportions of the radiant energy are converted to electric energy by thelarger bandgap semiconductor materials without excessive thermalizationand loss of energy in the form of heat, while the longer wavelength,lower energy portions of the radiation are transmitted to one or moresubsequent semiconductor materials with smaller bandgaps for furtherselective absorption and conversion of remaining radiation to electricalenergy.

Semiconductor compounds and alloys with bandgaps in the various desiredenergy ranges are known, but that knowledge alone does not solve theproblem of making an efficient and useful energy conversion device.Defects in crystalline semiconductor materials, such as impurities,dislocations, and fractures provide unwanted recombination sites forphotogenerated electron-hole pairs, resulting in decreased energyconversion efficiency. Therefore, high-performance, photovoltaicconversion cells comprising semiconductor materials with the desiredbandgaps, often require high quality, epitaxially grown crystals withfew, if any, defects. Growing the various structural layers ofsemiconductor materials required for a multi-bandgap, tandem,photovoltaic (PV) conversion device in a monolithic form is the mostelegant, and possibly the most cost-effective, approach.

Epitaxial crystal growth of the various compound or alloy semiconductorlayers with desired bandgaps is most successful, when all of thematerials are lattice-matched (LM), so that semiconductor materials withlarger crystal lattice constants are not interfaced with other materialsthat have smaller lattice constants or vice versa. Lattice-mismatching(LMM) in adjacent crystal materials causes lattice strain, which, whenhigh enough, is usually manifested in dislocations, fractures, waferbowing, and other problems that degrade or destroy electricalcharacteristics and capabilities of the device. Unfortunately, thesemiconductor materials that have the desired bandgaps for absorptionand conversion of radiant energy in some energy or wavelength bands donot always lattice match other semiconductor materials with otherdesired bandgaps for absorption and conversion of radiant energy inother energy or wavelength bands. Therefore, fabrication of devicequality, multi-bandgap, monolithic, converter structures is difficult,if not impossible, for some portions of the radiation frequency orwavelength spectrum.

This problem has been particularly difficult to solve in the infrared(IR) portion of the spectrum, where options for suitable, commerciallyavailable substrates on which to grow thin films with the necessarybandgaps for absorption and conversion of the infrared radiation toelectrical energy are very limited, and where compatible, i.e.,lattice-matched, semiconductor materials with the different bandgapsneeded to absorb and convert different portions of the infrared spectrumefficiently are also quite limited.

For example, the group III-V family of semiconductor alloys include someof the best materials for fabricating photovoltaic converters withbandgaps in a range of about 0.35 eV to 1.65 eV to absorb and convertinfrared (IR) radiation with wavelengths in a range of about 3.54 μm to0.75 μm. Group III-V alloys comprise combinations of binary compoundsformed from Groups III and V of the Periodic Table. These binarycompounds can be alloyed together into various ternary or quaternarycompositions to obtain any desired bandgap in the range of 0.35 eV to1.65 eV. These alloys also have direct bandgaps (i.e., no change inmomentum is required for an electron to cross the bandgap between thevalance band and the conduction band), which facilitate efficientabsorption and conversion of radiant energy to electricity. However,InP, which has a lattice constant of 5.869 Å (sometimes rounded to 5.87Å) and a bandgap of 1.35 eV, is one of only a few feasible, commerciallyavailable substrate materials with a lattice constant even close tothose lower bandgap Group III-V alloys i.e., InP-based or relatedternary and quaternary compounds. The lowest bandgap Group III-V alloythat can be lattice-matched to the 5.869 Å lattice constant of an InPsubstrate is Ga_(0.47)In_(0.53)As, which has a bandgap of about 0.74 eV,which leaves a significant range of lower frequency, longer wavelength(>1.67 μm), infrared (IR) radiation that cannot be absorbed andconverted to electricity in monolithic converters in which thesemiconductor absorption materials are lattice-matched to the substrate.

While the current unavailability of efficient and cost-effective solarphotovoltaic (SPV) converters, especially multi-bandgap, monolithic,converter devices, capable of absorbing and converting infrared (IR)radiation in wavelengths greater than 1.67 μm leaves substantial amountsof energy in the solar spectrum to remain unconverted to electricity, instate-of-the-art SPV's, it is an even greater problem forthermophotovoltaic (TPV) devices. Infrared (IR) radiation of wavelengthsgreater than 1.67 μm comprises a substantial amount of the energyradiated from blackbodies, and thermophotovoltaic (TPV) converters areintended to absorb and convert as much radiant energy from blackbodiesto electric power as possible. Therefore, solutions to these problems,especially if such solutions could enable fabrication of monolithicconverters with multiple bandgaps in infrared (IR) energy ranges, theywould facilitate capture of more electric energy from solar and/orblackbody radiation.

U.S. Pat. No. 5,479,032 issued to S. Forrest et al., teaches that one ormore ternary In_(x)Ga_(1-x)As alloys with x>0.53, i.3., with band-gapsless than 0.75 eV, can be grown epitaxially on an InP substrate by usingintervening, graded layers of InAs_(y)P_(1-y) between the InP substrateand the In_(x)Ga_(1-x)P (x>0.53) layers. However, those Forrest et al.,patent teachings, which were directed to pixel detection of nearinfrared radiation incident on a focal plane for telecommunicationsapplications, are not useful in SPV and TPV applications.

SUMMARY

Accordingly, a general object of the present disclosure is to provide amonolithic, multi-bandgap, photovoltaic converter for absorbing andconverting infrared (IR) radiation of multiple wavelengths toelectricity.

A more specific object of this disclosure is to provide a photovoltaicconverter with at least one bandgap less than 0.74 eV to absorb infraredradiation in wavelengths longer than 1.67 μm and convert it toelectricity.

An even more specific object of this disclosure is to provide a electricdevice quality, multi-bandgap, monolithic, photovoltaic converter thathas at least one lattice-matched (LM), double-heterostructure (DH) witha bandgap less than 0.74 eV to absorb infrared (IR) energy inwavelengths longer than 1.67 μm and convert it to electricity.

Another specific object of the disclosure is to provide a devicequality, multi-bandgap, monolithic, photovoltaic device with at leastone lattice-matched (LM), double-heterostructure (DH) with a bandgapless than 0.74 eV, which is not lattice-matched to an InP substrate, butincluding a lattice constant transition layer or layers, which istransparent to infrared radiation wavelengths longer than about 1.67 μm,positioned somewhere between such lattice-matched (LM),double-heterostructure (DH) and the InP substrate.

Still another object of this disclosure is to provide a lattice constanttransition layer or layers, which is transparent to infrared (IR)radiation wavelengths longer than about 1.67 μm, positioned between twosubcells in a multi-bandgap, monolithic device, where the two subcellsare not lattice-matched to each other and at least one of the subcellshas a bandgap, which is less than the bandgap of the other subcell andis less than 0.74 eV.

Another object of the present disclosure is to provide one or moresubcells with bandgaps less than 0.74 eV on an InP substrate.

Another object of the present disclosure is to provide a bifacial,monolithic, integrated, module (MIM) comprising multiple subcells, atleast one subcell of which absorbs and converts radiation wavelengthsless than 0.92 μm to electricity.

Another object of the present disclosure is to provide a bifacial,monolithic, integrated, module (MIM) comprising multiple subcells, atleast one subcell of which absorbs and converts radiation wavelengthsless than 1.67 μm to electricity.

Another specific object of this disclosure is to provide a method ofvoltage-matching a plurality of subcell circuits that have subcells withdifferent bandgaps less than or equal to 1.35 eV.

Additional objects, advantages, and novel features of the disclosure areset forth in part in the description that follows and will becomeapparent to those skilled in the art upon examination of the followingdescription and figures or may be learned by practicing the embodimentsdescribed herein. Further, the objects and the advantages of theembodiments described herein may be realized and attained by means ofthe instrumentalities and in combinations particularly pointed out inthe appended claims.

To achieve the foregoing and other objects and in accordance with thepurposes of the a present disclosure, as embodied and broadly describedherein, a method of one embodiment described herein may comprise growingone or more subcell(s) that has a lattice constant greater than 5.869 Å,either alone or in combination with other subcells, on an InP substrateby using a lattice constant transition material between the InPsubstrate and the subcell(s) that have the lattice constants greaterthan 6.869 Å. The lattice constant transition material can beInAs_(y)P_(1-y), where y is graded either continuously or in discretestepped increments from one (1) to a value at which the InAs_(y)P_(1-y)has a lattice constant that matches the lattice constant of at least oneof the subcells with a lattice constant greater than 5.869 Å. Thesubcell bandgap is lower than the bandgap of the InP substrate and lowerthan the bandgap of the InAs_(y)P_(1-y), lattice constant transitionmaterial. Additional subcells with even lower bandgaps can also beadded, and, if any of such additional subcells has an even greaterlattice constant that cannot be matched to the first subcell, then oneor more additional lattice constant transition layers can also be added.All of the subcells can be grown on only one side of the substrate(monofacial) or one or more subcells can be grown on the front side ofthe substrate while one or more other subcells can be grown on the backside (bifacial), using whatever lattice constant transition layers arenecessary to accommodate the subcell(s) on each side of the substrate.

Isolation layers can be used between subcells for independent electricalconnection of the subcells, although, in bifacial embodiments, thesubstrate can be insulating or semi-insulating to serve as an isolationlayer. Alternately, tunnel junctions can be used for intra-cell currentflow between subcells. Either the monofacial or bifacial subcellstructures can be made in monolithic, integrated, modules (MIMs), whichare particularly useful for voltage-matching a plurality of suchsubcells, although the bifacial embodiments are particularly suitablefor such MIM structures and voltage matching. On the other hand, themonofacial embodiments are particularly useful in ultra-thin devices inwhich the substrate is removed.

To achieve the foregoing and other objects and in accordance with thepurposes of the various embodiments broadly described herein,embodiments may also comprise a monolithic, multi-bandgap, photovoltaicconverter that has a first subcell comprising GaInAs(P) with a firstbandgap and a first lattice constant, a second subcell comprisingGaInAs(P) with a second bandgap and a second lattice constant, whereinthe second bandgap is less than the first bandgap and the second latticeconstant is greater than the first lattice constant, and further,wherein the second lattice constant is equal to a lattice constant of aInAs_(y)P_(1-y) alloy with a bandgap greater than the first bandgap, anda lattice constant transition material positioned between the firstsubcell and the second subcell, said lattice constant transitionmaterial comprising InAs_(y)P_(1-y) alloy with a lattice constant thatchanges gradually from the first lattice constant to the second latticeconstant.

In one embodiment, the first subcell is a lattice-matched,double-heterostructure, comprising homojunction layers of GaInAs(P) cladby InAs_(y)P_(1-y) cladding layers wherein the InAs_(y)P_(1-y) claddinghas a value for y in a range of o≦y<1, such the InAs_(y)P_(1-y) claddinglayers of the first subcell have a lattice constant equal to the firstlattice constant. The second subcell may be a lattice-matched,double-heterostructure comprising homojunction layers of GaInAs(P) cladby InAs_(y)P_(1-y) cladding layers, wherein the InAs_(y)P_(1-y) claddinghas a value for y in a range of o≦y<1, such that the InAs_(y)P_(1-y)cladding layer of the second subcell have a lattice constant equal tothe second lattice constant. Either a tunnel junction or an isolationlayer is also positioned between subcells. The InP substrate can bedoped with deep acceptor atoms to make the substrate more electricallyinsulating, and, in bifacial structures, this feature allows thesubstrate to serve as an electrical isolation between subcellspositioned on opposite sides of the substrate.

A plurality of the monolithic, multi-bandgap, photovoltaic converterscan also be grown on a common substrate in a monolithic, integrated,module (MIM), comprising the plurality of monolithic, multi-bandgap,photovoltaic converters, each of which comprises: (i) a first subcellwith a first bandgap and a first lattice constant; (ii) a second subcellwith a second bandgap and a second lattice constant, wherein the secondbandgap is less than the first bandgap and the second lattice constantis greater than the first lattice constant; and (iii) a lattice constanttransition material positioned between the first subcell and the secondsubcell, said lattice constant transition material having a bandgap atleast as large as the first bandgap and a lattice constant that changesfrom the first lattice constant to the second lattice constant. Eithermonofacial structures or bifacial structures can be grown in MIMconfigurations, but the bifacial structure is particularly suited to MIMapplications. The subcells in MIM structures can be isolated forindependent electrical connection, or tunnel junctions can be provided.Isolated, independently connected, subcells are particularly adapted forvoltage-matching in MIM structures. There can be more subcell stacks onone side of the substrate than the other to facilitate suchvoltage-matching, where the subcells on one side of the substrate arelower bandgap than subcell on the other side of the substrate.

The substrates can also be removed to provide ultra-thin photovoltaicdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate a plurality of embodiments of the presentdisclosure, and together with the descriptions serve to explain theprinciples of the present disclosure. In the drawings:

FIG. 1 is a diagrammatic illustration of the general, significantcomponents of a monofacial, inverted, multi-bandgap, monolithic,photovoltaic device with two lattice-matched (LM),double-heterostructure (DH) subcells grown on an InP substrate andseries connected, wherein the second subcell, which has a bandgap thatis less than the bandgap of the first subcell and is less than about0.74 eV, is lattice-mismatched (LMM) to the InP substrate, but is grownon a transparent, lattice constant transition layer positioned betweenthe two subcells to accommodate the lattice mismatch;

FIG. 2 is a more detailed cross-sectional view of the device of FIG. 1showing some of the auxiliary structures and components useful in anembodiment of the device;

FIG. 3 is a bandgap versus lattice parameter chart showing bandgap andlattice constant parameters of semiconductor materials used as examplesin the embodiments of FIGS. 1 and 2;

FIG. 4 is a detailed cross-sectional view of a monofacial, inverted,multi-bandgap, monolithic, photovoltaic device similar to FIGS. 1 and 2,but with the subcells isolated electrically for independent connection;

FIG. 5 is a simplified cross-sectional view of a photovoltaic deviceillustrating more subcells and graded transparent layers according toone or more embodiments described herein;

FIG. 6 is a diagrammatic illustration of a variation of the monolithic,multi-bandgap, photovoltaic converter similar to FIG. 1, but with thelattice constant transition layer positioned between the substrate andthe first subcell;

FIG. 7 is a bandgap versus lattice parameter chart showing bandgap andlattice constant parameters of the semiconductor materials used asexamples in the embodiment of FIG. 6;

FIG. 8 is a diagrammatic illustration of a variation of the monolithic,multi-bandgap, photovoltaic converter similar to FIG. 6, but with anadditional lattice constant transition layer and an additional subcelladded to the structure;

FIG. 9 is a diagrammatic illustration of a bifacial, buried substrateembodiment in which subcells are grown epitaxially on opposite faces ofthe substrate;

FIG. 10 is a bandgap versus lattice parameter chart showing bandgap andlattice constant parameters of the semiconductor materials used asexamples in the embodiment of FIGS. 9 and 11;

FIG. 11 is an illustration of a more complex, bifacial, monolithic,multi-bandgap, photovoltaic device;

FIG. 12 is an illustration of another more complex, bifacial,monolithic, multi-bandgap, photovoltaic device that is particularlyuseful for solar photovoltaic (SPV) converter applications;

FIG. 13 is a bandgap versus lattice parameter chart showing bandgap andlattice constant parameters of the semiconductor materials used asexamples in the embodiment of FIG. 12;

FIG. 14 is a cross-sectional view of a bifacial, monolithic integratedmodule (MIM);

FIG. 15 is a schematic diagram of an equivalent electric circuit showingthe voltage-matched electric subcell circuits of the bifacial MIM inFIG. 14;

FIG. 16 is a diagrammatic illustration of a monolithic, multi-bandgap,photovoltaic device similar to FIG. 2, but with an added stop-etch layerand with the structure mounted on a panel, heat sink, printed circuitboard, or other object; and

FIG. 17 is a diagrammatic view similar to FIG. 16, but with thesubstrate removed.

DETAILED DESCRIPTION

A schematic diagram of principle components of a monofacial embodimentof a low-bandgap, monolithic, multi-bandgap (tandem) photovoltaic (PV)converter 10 according to one embodiment is shown in FIG. 1 juxtaposedto a corresponding bandgap energy (E_(g)) profile. The diagram in FIG. 1illustrates a cross-section of the PV converter 10 profile in a mannerthat is conventional in the industry, i.e., not necessarily inproportion to actual sizes, because actual layer thicknesses are toosmall to illustrate in actual proportions. Additional structuralcomponents used to fabricate an example of the PV converter 10 of FIG. 1are illustrated in FIG. 2, which will be described in more detail below.

In the monofacial embodiment or approach illustrated in FIG. 1, all ofthe subcells, for example, subcells 22, 24 in FIG. 1, are grownepitaxially on only one side or face 25 of a substrate 26. Bifacialembodiments or approaches (not shown in FIG. 1), in which subcells aregrown epitaxially on opposite sides or faces of a substrate, will beillustrated in other figures and described below.

The monofacial PV converter 10 illustrated in FIG. 1 is designed withlow bandgap, Group III-V semiconductor alloy materials, especially forbandgaps below about 0.74 eV, where ternary GaInAs or AlInAs andquaternary (GaInAsP or AlGaInAs semiconductor alloys do not match thecrystal lattice constant of InP substrates 26. A quick reference to thelattice parameter versus bandgap chart in FIG. 3 shows that the crystallattice constant of InP is about 5.87 Å, as indicated by broken line 12,while the lowest possible bandgap for a Group III-V alloy with that samelattice constant of 5.87 Å is about 0.74 eV, which is provided by theternary alloy Ga_(0.47)In_(0.53)As, as indicated by the broken line 14.Lattice-matched (LM) materials refers to materials with latticeconstants that are either equal or similar enough that when thematerials are grown epitaxially, one or the other adjacent each other ina single crystal, any difference in size of crystalline structures ofthe respective materials is resolved substantially by elasticdeformation and not by inelastic relaxation, separation, dislocations,or other undesirable inelastic effects. A lattice-mismatch (LMM) isgenerally considered to occur when a second crystalline material beinggrown on a first crystalline material has a lattice constant that is notequal to the lattice constant of the first material and is notlattice-matched as described above. (The terms “lattice parameter” and“lattice constant” mean substantially the same thing and are often usedinterchangeably in the art and in this description.) Therefore, as shownby broken lines 16, 18 in FIG. 3, any Group III-V alloy with a bandgapless than about 0.74 eV will be a lattice-mismatched (LMM) with an InPsubstrate. Since a significant feature of this disclosure is to providea monolithic, multi-bandgap, photovoltaic (PV) converter with at leastone bandgap less than about 0.74 eV, such a lattice-mismatch has to bemitigated in order to avoid the adverse manifestations of lattice strainand stresses caused by such lattice-mismatch, such as dislocations,fractures, wafer bowing, rough surface morphologies, and the like.

Referring again to the exemplary monofacial, monolithic, multi-bandgap,photovoltaic (PV) converter 10 illustrated in FIG. 1, such mitigation oflattice-mismatch between a first Group III-V semiconductor subcell 22and a second Group III-V semiconductor subcell 24 with a differentbandgap below about 0.74 eV is provided by a lattice constant transitionlayer 20 that: (i) has graded (either distinctly stepped increments orcontinuously increasing) lattice constants, which span the differencebetween the respective lattice constants of the first and secondsubcells 22, 24; and (ii) is transparent to infrared wavelengths longerthan those absorbed by the first subcell 22. While a transparent latticeconstant transition layer 20, which is graded to have lattice constantsthat vary continuously from the lattice constant of the first subcell 22to the lattice constant of the second subcell 24, is satisfactory forthis purpose, a transparent lattice constant transition layer 20comprising, discrete or stepped changes in lattice constants might bepreferable. Dislocations in semiconductor crystals are undesirable,because they facilitate recombination of charge carriers (electron holepairs), which is deleterious to the electrical performance of asemiconductor device.

In one embodiment, a lattice constant transition layer 20 is a ternaryInAs_(y)P_(1-y) material in which the proportion of As is graduallyincreased, either continuously or in discrete increments as will bediscussed in more detail below. One significant feature is that theInAs_(y)P_(1-y) lattice constant transition layer 20 is transparent toinfrared (IR) radiation wavelengths longer than those absorbed by theternary Ga_(x)In_(1-x)As or optional quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) in the first subcell 22, so there isvirtually no loss of energy, or production of heat, in the latticeconstant transition layer 20.

The monofacial, monolithic, multi-bandgap, photovoltaic (PV) converter10 illustrated in FIG. 1 has an inverted structure with the activesubcell layers 22, 24, transparent lattice constant transition layer 20,and optional other layers (shown in FIG. 2, which will be described inmore detail below), grown epitaxially on one side 25 of a substrate 26.This structure is called inverted, because the radiation energy R entersthe converter 10 through the substrate 26, so it has to be transmittedthrough the substrate 26 before being absorbed and converted toelectricity by the subcells 22, 24. Therefore, as will be explained inmore detail below, the substrate 26 has to be transparent to all theincident radiation R so that none of the incident radiation R isabsorbed and thermalized or lost as heat before it reaches the subcells22, 24, where it can be converted to electricity. Likewise, the incidentradiation R transmitted by the substrate 26 should encounter the subcell22 with the highest bandgap before it encounters the subcell 24 with thelowest bandgap, because higher bandgap subcells will absorb only thehigher energy radiation (higher frequency and shorter wavelength) andconvert it to electricity while transmitting unabsorbed, lower energyradiation (lower frequency and longer wavelength). Therefore, any of theremaining lower energy incident radiation R that is not absorbed andconverted to electricity by the higher bandgap, first subcell 22 will betransmitted to the lower bandgap, second subcell 24, where at leastsome, if not all, of it can be absorbed and converted to electricity.The amount of the remaining, lower energy, incident radiation that canbe absorbed and converted to electricity by the second subcell 24 willdepend on the particular bandgap of the subcell 24 and the particularradiation wavelengths in such remaining, lower energy, incidentradiation. Of course, the lattice constant transition layer 20 has to betransparent to, and not absorptive of, the remaining, lower energy,incident radiation that is not absorbed by the first subcell 22 so thatall of such remaining, lower energy, incident radiation can reach thesecond subcell 24. Further, as will be explained in more detail below,additional subcells with different bandgaps can also be included inorder to optimize absorption and conversion of various incidentradiation energy levels or bands to electricity.

The back-surface reflector (BSR) or other spectral control element 28,which can also function as an electrode contact or lateral current flowelement, is deposited on the second subcell 24, as will be described inmore detail below. A spectral control layer 30 would usually bedeposited on the front side 27 of the substrate 26 either to minimizereflection of incident radiation R, e.g., an anti-reflective coating(ARC), as is well-known to persons skilled in the art, especially forSPV converter applications, or to reflect all incident radiation R withwavelengths lower than those absorbable by the lowest bandgap subcell24, especially for the TPV converter applications used for generatingelectricity and not heat. These structures and functions will bediscussed in more detail below. The terms front and back, as used inthis description, relate to the direction in which incident radiationpropagates into and through a device or layers in a device. Therefore,radiation is incident first on the front of a device or layer andpropagates toward the back of the device or layer.

In converter 10, substrate 26 comprises InP, because, as explainedabove: (i) InP has a lattice constant (5.87 Å), which is one of a fewcommercially available bulk, single crystal materials that are close tothe lattice constants of Group III-V alloys that have bandgaps less than0.74 eV (for absorbing infrared radiation wavelengths longer than about1.67 μm); (ii) InP has a bandgap of about 1.35 eV (see FIGS. 1 and 3),thus does not absorb, and is transparent to, infrared radiationwavelengths longer than 0.93 μm; (iii) InP can be doped to be highlyresistive and thereby function as an insulator or semi-insulator, asdescribed in more detail below; (iv) Lattice-mismatch between InP andInAs_(y)P_(1-y) or GaInAs(P) materials, which have lower bandgaps thanInP and are used extensively in this disclosure as explained below, isin compression rather than tension, so lattice-mismatchedInAs_(y)P_(1-y) or GaInAs(P) grown on InP are not so likely to developfissures or crack; and (v) Bulk InP crystals are less expensive thanInAs and GaSb. Therefore, the InP substrate 26 is suitable in amonofacial, inverted PV converter 10 structure for any application inwhich the incident radiation R to be converted to electric energy has0.93 μm and longer wavelengths, such as thermophotovoltaic (TPV) cellsand some solar photovoltaic (SPV) cells as well as infrared detectordevices and multi-bandgap infrared (IR) LED's. However, InP issusceptible to free carrier absorption of energy, which results inenergy being lost in the form of heat. To minimize or prevent such freecarrier absorption of energy, the InP substrate can be doped with deepacceptor atoms, such as iron (Fe) or chromium (Cr), to pin the Fermilevel deeply within the bandgap, which makes the InP act more like aninsulator or semi-insulator.

Subject to accommodations for a contact, buffer, cladding, opticalcontrol elements, and/or other auxiliary layers (not shown in FIG. 1),which will be described in more detail below, the first subcell 22 isdeposited on substrate 26 with a bandgap E_(g1) designed to absorb thefirst desired wavelength or frequency band of the incident radiation Rand convert such absorbed radiation to electricity. In one embodiment,this first subcell 22 is a lattice-matched (LM), double-heterostructure(DH), InP/Ga_(x)In_(1-x)As or InP/Ga_(x)In_(1-x)As_(y)P_(1-y) with adesired bandgap E_(g1), somewhere in a range that is less than the 1.35eV bandgap of the InP substrate 26. This first subcell 22 in FIG. 1 maybe grown epitaxially and is lattice-matched to the InP substrate 26.Please note that “lattice-matched” when used in the context of a“lattice-matched, double-heterostructure” for a subcell generally meansthat the semiconductor materials within the subcell itself arelattice-matched to each other. Therefore, a subcell can be alattice-matched, double-heterostructure, while such subcell may or maynot be lattice-matched to a substrate or to another layer or material inthe device that is not part of the subcell.

In one embodiment, subcell 22 lattice-matched to the InP substrate 26comprises InP/Ga_(0.47)In_(0.53)As with a bandgap of about 0.74 eV,because, as shown by the lines 12, 14 in FIG. 3, Ga_(0.47)In_(0.53)As isthe lowest bandgap Group III-V alloy that has the same lattice constantas the InP substrate 26. Therefore, a InAs_(y)P_(1-y) lattice constanttransition layer 20 can make a transition from the lattice constant ofInP (about 5.87 Å) to a lattice constant matching a Ga_(x)In_(1-x)Asalloy with a bandgap as low as about 0.52 eV, i.e., to a latticeconstant as high as about 5.968 Å(see lines 15, 17 in FIG. 3), and stillbe transparent to all infrared wavelengths that are longer than thoseabsorbed by the 0.74 eV bandgap of the first subcell 22 (see line 14 inFIG. 3). Of course, the desired bandgap for the second subcell 24 couldalso be anywhere between 0.74 eV and 0.52 eV, in which case theInAs_(y)P_(1-y) lattice constant transition layer 20 can be formulatedto provide a back surface with whatever lattice constant is needed onwhich to grow the desired Ga_(x)In_(1-x)As_(y)P_(1-y) that has such adesired bandgap.

An example second subcell 24 for use in conjunction with a first subcell22 described above, therefore, can be a quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) or a ternary Ga_(x)In_(1-x)As with a bandgapas low as 0.52 eV. In one embodiment, an example second cell 24comprises a lattice-matched, double-heterostructureInAs_(y)P_(1-y)/Ga_(x)In_(1-x)As with a bandgap 19 of 0.55 eV and alattice constant 21 of about 5.952 Å.

The lattice constant transition layer 20, as mentioned above, graduallymakes a transition from the lattice constant of the first subcell 22 tothe lattice constant of the second subcell 24, while remainingsubstantially transparent to all infrared radiation wavelengths that arenot absorbed by the first subcell 22, as illustrated by the example PVconverter 10 of FIG. 1. In that specific example, the first subcell 22has a bandgap (E_(g1)) 14 of about 0.74 eV and lattice constant of about5.87 Å, while the second subcell 24 has a bandgap (E_(g2)) 19 of about0.55 eV and lattice constant of about 5.952 Å, as explained above.Therefore, the lattice constant transition layer 20 has to make atransition of lattice constants gradually from about 5.87 Å to about5.952 Å. As shown in FIGS. 1 and 3, adding As to InP to produceInAs_(y)P_(1-y) increases the lattice constant of the InAs_(y)P_(1-y)from about 5.87 Å to about 5.952 Å without decreasing the bandgap of theInAs_(y)P_(1-y) to a level below the 0.74 eV Ga_(x)In_(1-x)As (x=0.47)of the first subcell 22. Therefore, the InAs_(y)P_(1-y) lattice constanttransition layer 20 remains transparent to all of the remaining infraredradiation R that is not absorbed in the first subcell 22 so that itallows all of such remaining infrared radiation to reach the secondsubcell 24.

As also mentioned above, such graded transition of the InAs_(y)P_(1-y)lattice constant transition layer 20 from the lattice constant of thefirst subcell 22 (e.g., 5.87 Å) to the lattice constant of the secondsubcell 24 (e.g., 5.952 Å) can be done by increasing the proportion ofAs on a gradual continuous basis or, in incremental discrete steps asillustrated by line 23 in the bandgap chart in FIG. 1. The steppedlattice constant transition 23 illustrated in FIG. 1 seems to providebetter experimental results than gradual, continuous grading.

A more specific example of the monofacial PV converter 10 of FIG. 1 withauxiliary layers useful in actual implementation of such a device forhigh quality performance characteristics is shown diagrammatically inFIG. 2. Again, the thicknesses of the various layers are not illustratedin actual size or thickness proportions in relation to each other.

The substrate 26 may be InP doped with a deep acceptor element, such asFe, (sometimes denoted as InP:Fe or as (Fe) InP) to trap electrons andthereby suppress or prevent free carrier absorption. The substrate 26can be semi-insulating for isolation or p-type for conducting, asdesired for a particular application, and other layers and componentsare designated as either n-type or p-type, accordingly to provide then/p junctions 34, 48 needed to convert the incident radiation R toelectricity in the subcells 22, 24, respectively. However, p/n junctionswould also work, as is understood by persons skilled in the art, sothese n-type and p-type designations could be reversed by substitutingdonor dopants for acceptor dopants and vice versa, which would beconsidered equivalent for purposes of this disclosure.

While the subcells 22, 24 can be simple shallow homojunctions, thisembodiment is particularly conducive to the more efficient,lattice-matched, double-heterostructure subcells 22, 24 illustrated inFIG. 2. Specifically, the example first subcell 22 illustrated in FIG. 2has a n/p homojunction 34 formed by a p-type Ga_(x)In_(1-x)As base layer38 grown epitaxially on an n-type Ga_(x)In_(1-x)As emitter layer 36, allof which is sandwiched between front and back cladding layers 40, 42 ofn-type InP and p-type InP, respectively. The InP in the cladding layers40, 42 is a different compound than the Ga_(x)In_(1-x)As in thehomojunction layers 36, 38, but it has the same lattice constant as theGa_(x)In_(1-x)As. Therefore, the first subcell 22 is a lattice-matched,double-heterostructure. The cladding layers 40, 42 passivate danglingbonds at terminated Ga_(x)In_(1-x)As crystal structures at the front oflayer 36 and at the rear of layer 38, which otherwise function, at leastto some extent, as unwanted recombination sites for minority carriers inthe Ga_(x)In_(1-x)As. Also, the band offsets between the InP (bandgap of1.35 eV) and the Ga_(x)In_(1-x)As (bandgap of 0.74 eV in this example)repel minority carriers away from the InP/Ga_(x)In_(1-x)As interface,which further reduces such unwanted recombination of minority carriers.Therefore, a clad subcell, such as the lattice-matched (LM),double-heterostructure (DH), InP/Ga_(0.47)In_(0.53)As or optionalInP/Ga_(x)In_(1-x)As_(y)P_(1-y) subcell 22 described above, is moreefficient in converting radiant energy to electricity than non-cladsubcells. This passivation and confinement of minority carriers by thecladding layers 40, 42 is possible in the monolithic, multi-bandgap PVconverter structures, because the cladding material, InP, has the samelattice constant (5.869 Å) as, and a higher bandgap than, thehomojunction cell material, Ga_(x)In_(1-x)As orGa_(x)In_(1-x)As_(y)P_(1-y) (Ga_(x)In_(1-x)As_(y)P_(1-y) islattice-matched to InP when y≈2.2x).

The second subcell 24 may also be a lattice-matched,double-heterostructure comprising a homojunction 48 formed by n-type andp-type layers 50, 52 of either ternary Ga_(x)In_(1-x)As or quaternaryGa_(x)In_(1-x)As_(y)P_(1-y), but its lattice constant is larger than thelattice constant of the first subcell 22 and of the InP substrate 26, asexplained above. Consequently, the second subcell 24 islattice-mismatched (LMM) in relation to the InP substrate 26 and firstcell 22, and it cannot be clad with InP. However, as explained above inrelation to the lattice constant transition layer 20, InAs_(y)P_(1-y)can be formulated to have the same lattice constant as theGa_(x)In_(1-x)As or Ga_(x)In_(1-x)As_(y)P_(1-y) homojunction layers 50,52. Therefore, the passivation and confinement cladding layers 54, 56 ofthe second subcell 24 comprise InAs_(y)P_(1-y) that is lattice-matchedto the Ga_(x)In_(1-x)As or Ga_(x)In_(1-x)As_(y)P_(1-y) homojunctionlayers 50, 52 to form the lattice-matched, double-heterostructure ofthat subcell 24.

Prior to growing the first subcell 22, a buffer layer 32 of n-InP about300 Å thick is deposited first on a surface 25 of the InP substrate 26to begin an epitaxial InP growth layer, if needed. If the InP substrate26 is doped with a deep acceptor to be electrically insulating orsemi-insulating as explained above, then provisions have to be made fora front electrical contact 29 and a conductive layer 33 foraccommodating lateral flow of current produced by the subcells 22, 24 toor from the contact 29. Such a conductive layer 33 could be, forexample, heavily n-doped InP or any other heavily doped material that islattice-matched to the InP substrate 26 as well as transparent to allradiation wavelengths that are transmitted by the InP substrate 26.Then, the first subcell 22 comprising the lattice-matched,double-heterostructure of n-Ga_(0.47)In_(0.53)As/p-Ga_(0.47)In_(0.53)Ashomojunction layers 36, 38 between the two cladding layers 40, 42 ofn-InP and p-InP, respectively. As is well-known in the art,semiconductor materials are usually doped with small amounts of elementsfrom an adjacent group of the Periodic Table of the Elements to providethe majority carriers. Therefore, an appropriate donor dopant for theGroup III-V semiconductor alloy used can be, for example, sulphur (S)from Group VI, and appropriate acceptor dopant can be, for example, zinc(Zn) from Group II. The InP buffer layer 32 grown epitaxially on the InPsubstrate 26 in this example is heavily (10⁻¹⁸-10⁻²⁰ cm⁻³) n-type dopedwith sulfur (S). Then, the InP front cladding layer 40 is grownepitaxially on the buffer layer 32 to a thickness of about 0.01-0.1 μm,but it is more lightly doped n-type with, for example, S to a dopantlevel of about 10¹⁶-10²⁰ cm⁻³. The Ga_(0.47)In_(0.53)As homojunctionlayers 36, 38, which lattice-match the InP substrate 26, buffer layer32, and cladding layer 40, are grown epitaxially. Therefore, the bandgapof the first subcell 22 is about 0.74 eV, which absorbs portions of theincident radiation R with wavelengths of about 1.67 μm and less, asexplained above, although other values of x and other formulations wouldalso work in alternate embodiments. Lattice-matching quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) is also possible. The emitter layer 36 ofsubcell 22 is grown epitaxially to a thickness in a range of about0.1-10 μm, and is doped n-type with, for example, S to a dopant level ina range of about 10¹⁶-10²⁰ cm⁻³. The base layer 38 is then grownepitaxially to a thickness of about 0.01-10 μm, and doped p-type tocreate the n/p junction 34. The p-type dopant, such as Zn in thisexample, is at a dopant level of about 10¹⁶-10²⁰ cm⁻³. To complete thelattice-matched, double-heterostructure, first subcell 22, the backcladding layer 42 is grown epitaxially on the base layer 38 to athickness of about 0.01-0.1 μm, and is p-type doped, for example, withZn, to a dopant level of about 10¹⁶-10²⁰ cm⁻³.

Each of the buffer layer 32, conductive layer 33, and/or cladding layer40 can all serve any one or more of these functions, individually ortogether. Therefore, instead of the three distinct layers 32,33,40 shownin FIG. 2, one or two layers could serve those same functions, ifdesired.

The subcells 22, 24 can be electrically connected together in series, orthey can be electrically isolated from each other, as will be describedin more detail below. For a monolithic, multi-bandgap, PV device 10 inwhich the subcells 22, 24 are series connected, a tunnel junctioncomprising a layer 44 of heavily p-doped Ga_(0.47)In_(0.53)As orGa_(x)In_(1-x)As_(y)P_(1-y) followed by a heavily n-dopedGa_(0.47)In_(0.53)As or Ga_(x)In_(1-x)As_(y)P_(1-y) layer 46 isdeposited and grown epitaxially on the back cladding layer 42 of thefirst subcell 22 to facilitate low-resistive current flow in an ohmicmanner between the first subcell 22 and the second subcell 24. Again, ifhomojunction layers 36, 38 of subcell 22 comprise Ga_(0.47)In_(0.53)As,as discussed above, then it may be that x=0.47 in the Ga_(x)In_(1-x)Asof the tunnel junction layers 44, 46 in order to lattice-match them withthe underlaying InP and Ga_(0.47)In_(0.53)As layers described above,although other values of x and other formulations would also work.Tunnel junctions are well-known in the art, but, for purposes of thisembodiment, each tunnel junction layer 44, 46 can be about 0.01-0.1 μmthick and doped to a level of about 10⁻¹⁸-10⁻²⁰ cm⁻³. Alternativemonolithic, multi-bandgap, PV converters with the subcells 22, 24isolated electrically from each other will be described below.

The transparent, lattice constant transition layer 20 comprisinggradually increasing lattice constants is deposited and grownepitaxially on the Ga_(x)In_(1-x)As or Ga_(x)In_(1-x)As_(y)P_(1-y)tunnel junction layer 46 in order to make the transition from thelattice constant of the InP substrate 26 and intervening layersdescribed above to a lattice constant that matches the Ga_(x)In_(1-x)Asor Ga_(x)In_(1-x)As_(y)P_(1-y) of the second subcell 24, which isformulated to provide a desired bandgap E_(g2), as described above.According to one embodiment, the bandgap E_(g2) is less than the bandgapE_(g1) of the first subcell 24 in the monofacial, inverted PV converterembodiment 10 of FIGS. 1 and 2 for the reasons explained above.InAs_(y)P_(1-y) is used for this lattice constant transition layer 20,because it can be formulated to lattice match the lower bandgap E_(g2)of the Ga_(x)In_(1-x)As of Ga_(x)In_(1-x)As_(y)P_(1-y) of the secondsubcell 24, while it also remains transparent to the longer infraredradiation R wavelengths that are not absorbed by the higher bandgapE_(g1) material of the first subcell 22. This feature is important inorder to ensure that substantially all of the longer wavelengthradiation R, which is not absorbed in the first subcell 22, reaches thesecond subcell 24.

To form the lattice constant transition layer 20, (As) is added to agrowing layer of InP in increasing proportions so that the proportion ofarsenic (As) increases in the resulting InAs_(y)P_(1-y) material, whichincreases the lattice constant of the InAs_(y)P_(1-y). As mentionedabove, this change can be accomplished continuously, or the changes inproportions be made in incremental steps. In the InAs_(y)P_(1-y) of thelattice constant transition layer 20 of this example PV converter 10, yvaries from zero (where it lattice-matches the Ga_(0.47)In_(0.53)As ofthe first subcell 22) to about 0.44, where it lattice-matches to theGa_(x)In_(1-x)As of the second subcell 24, in which x≈0.26 and theconsequent bandgap E_(g2) is about 0.55 eV. That example bandgapEg₂=0.55 eV enables the second subcell 24 to absorb infrared radiation Rwith wavelengths up to about 2.25 μm. In general, the lattice-matchingcondition of GaxIn1-xAs to InAsyP1-y occurs when the crystal lattices ofthe epi-layers are fully relaxed, which is where y≈2.143x.

Of course, as mentioned above, the Ga_(x)In_(1-x)As of the secondsubcell 24 can have x equal to some other value for a different desiredbandgap E_(g2), and the y in the InAs_(y)P_(1-y) of the lattice constanttransition layer 20 can be varied or customized accordingly to make thenecessary corresponding lattice constant transition. Also, as mentionedabove, either or both of the subcell materials and/or the latticeconstant transition materials could be quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) with the x and y values customized todesired bandgaps and lattice constants within the physical constraintsillustrated by the bandgap vs. lattice parameter chart of FIG. 3.

As explained above and shown in FIG. 2, the second subcell 24 comprisesa lattice-matched, double-heterostructure of n-type InAs_(y)P_(1-y)front cladding layer 54, n-type Ga_(x)In_(1-x)As orGa_(x)In_(1-x)As_(y)P_(1-y) emitter layer 50 and p-type Ga_(x)In_(1-x)Asor Ga_(x)In_(1-x)As_(y)P_(1-y) base 52 to form the homojunction 48, andp-type InAs_(y)P_(1-y) back cladding layer 56, all grown epitaxially onthe lattice constant transition layer 20. As explained above for thefirst subcell 22, the cladding layers 54, 56 confine and passivate thefront and back surfaces of the homojunction layers 50, 52 to preventrecombination of minority carriers. The thicknesses and doping levels ofthe subcell 24 layers 50, 52, 54, 56 can be similar to those describedabove for the first subcell 22.

A back surface spectral control element 28, which can also be used as aback electrical contact layer, can be deposited onto the back claddinglayer 56 or onto an additional contacting layer (not shown) disposedatop the back cladding layer 56. The nature of the back surface spectralelement 28 may depend on the application of the device 10. For example,if the device 10 is a SPV or TVP, the sole purpose of which is toconvert radiation to electricity, then the back surface spectral elementmay comprise a reflector to reflect any remaining, unabsorbed radiationfrom the second subcell 24 back through the subcells 24, 22. Some ofsuch reflected radiation could be absorbed in this second pass throughthe subcells, but most of it will continue propagating all the way backthrough the substrate 26 toward whatever radiator source (not shown)produces the incident radiation R in the first place. Adding suchunabsorbed, back-reflected, radiation energy back into the radiatorsource may enable the radiator source to use such back-reflected energyin the production of new incident radiation R for conversion toelectricity in the converter 10. This feature is particularlyappropriate for TPV configurations of converter 10 that are applied toconvert infrared radiation R produced by a blackbody infrared radiationsource (not shown) to electricity. Any radiation reflected back into theblackbody radiator adds energy to the blackbody radiator and therebytends to raise the temperature of the blackbody radiator, which causesthe blackbody radiator to produce more blackbody infrared radiation forthe converter 10. Therefore, such back-reflected radiation can help theblackbody radiator to produce more incident radiation R for the device10 without having to use so much fuel.

On the other hand, some devices 10 are used both for producingelectricity and gathering heat for an environment. In thoseapplications, the back surface spectral control element 28 may be amaterial that is transparent to remaining infrared radiation that is notabsorbed by the second subcell 24 so that such remaining infrared can beused as heat someplace behind the device 10.

If the layer 28 is a back surface reflector (BSR), there can be severaladvantages to designing the last (second) subcell 24 with only one-halfof its normal thickness, i.e., one-half the thickness that would berequired for full absorption of radiation in the wavelengths thatcorrespond to the bandgap, because any unabsorbed radiation will bereflected by the BSR 28 back into the last subcell 24. The advantages ofthis kind of design include an enhanced photocurrent, higher operatingvoltage, and thinner structure that requires less growth time andprovides easier device processing. Regardless of its opticalcharacteristics, as described above, the layer 28 can also be a backsurface electrical contact. Therefore, it may be electricallyconductive. An optional, additional metallic contact 45 can also be usedon the conductive layer 28 for making an electrical connection, ifdesired.

The design of the front surface spectral control element 30 on the frontsurface 27 of the substrate 26 may also depend on usage of the device10. For example, if the device 10 is to be used only for producingelectricity from blackbody radiation, the front surface spectral controlelement 30 may be a coating layer that transmits only shorter wavelengthincident radiation R that can be absorbed and converted to electricityby the subcells 22, 24 and that reflects all longer wavelength incidentradiation R back into the blackbody radiator (not shown) for recoveryand re-use. On the other hand, if the device 10 is to be used both forproducing electricity and heat for an environment, then the frontsurface spectral control element 30 may be an antireflective coating toenhance transmission of all the incident radiation R into the device 10.

As mentioned above, the monofacial PV converter 10 described above andillustrated in FIGS. 1 and 2 is configured with the subcells 22, 24connected electrically in series facilitated by the tunnel junctionlayers 44, 46. An alternate embodiment monofacial, low-bandgap,monolithic, multi-bandgap, PV converter 110 is shown in FIG. 4 with muchthe same first subcell 22, second subcell 24, and substrate 26structures and materials described above for the PV converter 10, butwith the subcells 22, 24 isolated electrically from each other. Theelectrical isolation instrumentality in the PV converter 110 isillustrated as a discrete electrical isolation layer 39 positionedbetween the first subcell 22 and the second subcell 24. However, suchelectrical isolation function could be incorporated into othercomponents, such as into the graded lattice constant transition layer20, as will be explained below.

There are a number of reasons that such electrical isolation of thesubcells 22, 24 may be desirable in some applications. For example, asmentioned above, current flow through series connected subcells 22, 24is limited by the lowest photocurrent producing subcell. Therefore, forseries connected subcells, a number of subcell design factors, such asbandgaps, thicknesses, doping concentrations, and the like are used tooptimize the operating characteristics of the series connected subcells22, 24, so that electric power production from the tandem combination ismaximized. In some designs and applications, however, more efficientconversion of radiant energy to electricity can be accomplished byextracting electric power from the individual subcells 22, 24 separatelyor independently, or, in some applications, to design the subcells 22,24 for voltage matching. Such voltage matching techniques with subcellsin other devices will be discussed in more detail below in relation tomonolithic, integrated module (MIM) devices.

To isolate the subcells 22, 24 electrically from each other, there hasto be some material between them that inhibits electric current flowbetween the subcells 22, 24. However, such electrical isolation materialcannot interfere with radiation transmission from one subcell 22 to theother subcell 24. In the PV converter 110 of FIG. 4, a discreteisolation layer 39 is shown positioned between the first subcell 22 andthe lattice constant transition layer 20, although it could bepositioned between the lattice constant transition layer 20 and thesecond subcell 24.

An isolation material for isolation layer 39 can be fabricated in anumber of ways. One such approach is to fabricate the isolation layer 39with a high-resistivity semiconductor material that has a high enoughbandgap to be transparent to the longer wavelength radiation that is notabsorbed in the first subcell 22 and is being transmitted to the secondsubcell 24. Another such approach is to form the isolation layer 39 asan isolation diode, which, of course, may also be transparent to theradiation being transmitted from the first subcell 22 to the secondsubcell 24. Also, such high-resistivity material or isolation diodematerial has to be lattice-matched to the materials in front and in backof it, which, in the position of isolation layer 39 shown in FIG. 4, hasto be lattice-matched to the first cell 22 and substrate 26.

As mentioned above, InP doped with a deep acceptor element, such as Feor Cr, is a high-resistivity material and has a bandgap (1.35 eV) thatmakes it transparent to all radiation that is not absorbed by the firstsubcell 22. It is also lattice-matched to the InP substrate 26 and tothe ternary Ga_(0.47)In_(0.53)As or quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) of the first subcell 22. Therefore, deepacceptor-doped InP can be used as the high-resistivity, isolation layer39. Such deep acceptor-doping of other lattice-matched semiconductormaterials, such as ternary Ga_(x)In_(1-x)As, quaternaryGa_(x)In_(1-x)As_(y)P_(1-y), or even AlGaInAs in some circumstances,with high enough bandgaps to be transparent to the radiation beingtransmitted, could also be used to provide suitable high-resistivitymaterials for the isolation layer 39.

An isolation diode for isolation layer 39 can be provided by one or moredoped junctions, such as an n-p junction or n-p-n junctions with highenough reverse-bias breakdown characteristics to prevent current flowbetween the subcells 22, 24. Again, lattice-matched semiconductormaterials, such as InP, Ga_(x)In_(1-x)As or Ga_(x)In_(1-x)As_(y)P_(1-y),or even AlGaInAs, can be doped to provide an isolation diode structurefor isolation layer 39.

While a discrete isolation layer 39 is shown in the PV converter 110 ofFIG. 4, it is possible to dope the lattice constant transition layer 20to also function as an isolation layer between the two subcells 22, 24.The InAs_(y)P_(1-y) of the lattice constant transition layer 20 can alsobe doped with a deep acceptor element, such as Fe or Cr, to make ithighly-resistive, or discrete sublayers of the InAs_(y)P_(1-y) can ben-p or n-p-n doped to form an isolation diode structure.

Of course, with each subcell 22, 24 isolated electrically from eachother, some additional provisions for electrical contacts are necessaryto extract electric power independently from each subcell 22, 24.Persons skilled in the art will be able to design myriad structures forsuch contacts, once they understand the principles described in thisdisclosure. The example additional contacts 27, 42 for this purpose areshown fabricated on lateral current flow layers 39, 41 respectively.Such lateral current flow layers 39, 41 are lattice-matched to theirrespective subcells 22, 24 and should be transparent to radiation beingtransmitted from the first subcell 22 to the second subcell 24. Heavilydoped GaIn_(1-x)As_(y)P_(1-y) with 0≦x≦1 and 0≦y≦1 as necessary forlattice matching and transparency can be used for these lateral currentflow layers 39, 41.

While the series connected PV converter 10 and isolated or independentlyconnected PV converter 110 described above are illustrated with only twosubcells 22, 24, and only one lattice constant transition layer 20between them, any number of subcells with any number of lattice constanttransition layers can be included in a monolithic, multi-bandgap,optoelectronic device according to alternate embodiments. To illustratethis principle, a more complex monolithic, multi-bandgap, PV converter112 is illustrated in FIG. 5.

In the PV converter 112, an arbitrary number (five) subcells 114, 116,118, 120, 122 are illustrated with arbitrary bandgapsE_(g1)>E_(g2)>E_(g3)>E_(g4)>E_(g5). The substrate 124 is InP, and thefirst and second subcells 114, 116 both have bandgaps E_(g1), E_(g2)that can be ternary GaInAs or quaternary GaInAsP and are lattice-matchedto the InP substrate 124 (see, e.g., lines 130, 12, 14 in FIG. 3). Thesefirst and second subcells 114, 116 may be both lattice-matched (LM),double-heterostructures (DH) with junctions comprising n-type and p-typeternary Ga_(x)In_(1-x)As or quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) cladwith n-type and p-type layers of InP, as described above for the otherPV converter embodiment 10. The third and fourth subcells 118, 120 areLM, DH ternary or quaternary GaInAs(P) with bandgaps E_(g3), E_(g4) thatcan be lattice-matched to each other, but not to the InP substrate (see,e.g., lines 12, 132, 134, 136 in FIG. 3). Therefore, a lattice constanttransition layer 126 of graded InAs_(y)P_(1-y) is used to make thetransition from the second subcell 116 to the lattice-mismatched subcell118. A fifth LM, DH subcell 122 of ternary or quaternary GaInAs(P) has abandgap E_(g5) that cannot be lattice-matched to the fourth subcell 120.Therefore, another lattice constant transition layer 128 is provided tomake the transition from the fourth subcell 120 to thelattice-mismatched fifth subcell 122.

As mentioned above, the numbers and combinations of subcells and latticeconstant transition layers as well as the specific example bandgapvalues shown in the PV converter 112 of FIG. 5 are selected arbitrarilyfor illustrative purposes. The only requirement is that the incidentradiation reaches the subcells in order of decreasing bandgaps, so thatthe shorter wavelength radiation is absorbed and converted toelectricity by higher bandgap subcells that will transmit unabsorbed,longer wavelength radiation to the next subcell(s). Other details, suchas buffer layers, tunnel junction or isolation layers, contacts, opticcontrol layers, etc., for fabricating a working PV converter can besimilar to those described above for either the series connected subcellembodiments 10 of FIGS. 1 and 2 or the independently connected subcellembodiment 110 of FIG. 4.

Now, as illustrated in another alternative inverted, monofacial,multi-bandgap, PV converter 140 in FIG. 6, the positions of thetransparent lattice constant transition layer 20 and the first subcell22 positions can be reversed from their positions shown in the FIG. 1embodiment 10. Specifically, the lattice constant transition layer 20can be grown epitaxially on the InP substrate 26 by gradually addingmore and more As to the growing InAs_(y)P_(1-y) lattice constanttransition layer 20, as described above, until a desired latticeconstant is attained for a desired Ga_(x)In_(1-x)As orGa_(x)In_(1-x)As_(y)P_(1-y) semiconductor material with a desiredbandgap to be grown on the InP substrate 26. As explained above, thedesire bandgap is chosen for absorbing and converting infrared radiationR of a desired wavelength or frequency band to electricity.

For example, but not for limitation, if it is desired to have the firstsubcell 22 in the PV converter 140 of FIG. 6 absorb and convert infraredradiation of at most 1.77 μm wavelength to electricity and to have thesecond subcell 24 absorb and convert infrared radiation in the range of1.77 μm to 2.14 μm to electricity, the first subcell 22 would need abandgap of about 0.70 eV, and the second subcell 24 would need a bandgapof about 0.58 eV. Therefore, an appropriate lattice constant transitionlayer 20 can be InAs_(y)P_(1-y) with a gradually increasing proportionof As until an InAs_(y)P_(1-y) semiconductor material having a latticeconstant of about 5.94 Å and a bandgap of about 0.90 eV, as illustratedin FIG. 7 by broken lines 60, 62, respectively. Therefore, a latticeconstant transition layer 20 with those criteria will provide atransition of lattice constant 12 from the 5.87 Å of the InP substrateto the 5.94 Å of the terminal InAs_(y)P_(1-y) material in the latticeconstant transition layer 20. With a terminal bandgap of 0.90 eV, thelattice constant transition layer 20 is transparent to infraredradiation (IR) with wavelengths longer than about 1.38 μm.

The first subcell 22 with the example desired 0.70 eV bandgap can thenbe a lattice-matched (LM), double-heterostructure (DH) of, for example,InAs_(y)P_(1-y)/Ga_(x)In_(1-x)As_(y)P_(1-y) with the same latticeconstant, 5.94 Å, as the terminal InAs_(y)P_(1-y) of the latticeconstant transition layer 20 (see broken line 60 in FIG. 7). TheGa_(x)In_(1-x)As_(y)P_(1-y) base of the subcell 22 can be formulated tohave the desired bandgap of 0.70 eV (see broken line 64 in FIG. 7 andcorresponding line 64 in FIG. 6), so it will absorb and convert 1.77 μmand shorter radiation R to electricity, but it will transmit and notabsorb virtually all the incident infrared radiation (IR) that is longerwavelength than 1.77 μm. Such formulation of appropriate proportions ofGroup III-V elements in quaternary alloys, such as theGa_(x)In_(1-x)As_(y)P_(1-y) in this example, to achieve certain desiredbandgap characteristics, such as the 0.70 eV in this example, iswell-known and within the capabilities of persons skilled in the art,thus need not be explained in detail here to enable persons skilled inthe art to understand and practice these embodiments.

The second subcell 24 in this example can be formulated with alattice-matched (LM), double-heterostructure (DH) with the same latticeconstant of 5.94 Å as the first subcell 22 and still have a bandgap aslow as 0.58 eV (see broken lines 60, 66 in FIG. 7). Therefore, if it isdesired to formulate the second cell 24 to absorb and convert as much ofthe infrared radiation R, which passed through the first cell 22, aspossible, and still be lattice-matched to the first cell 22, thenGa_(x)In_(1-x)As with a bandgap of 0.58 eV can be used. This examplesecond cell 24, with its 0.58 eV bandgap 66, would absorb and convertinfrared radiation R of 2.14 μm wavelength and shorter to electricity.Of course, other auxiliary layers, such as buffers, cladding, tunneljunction or isolation layers, contacts, and antireflective or opticalcontrol layers can be used to make this structure a functioning device,as explained above, and as would be understood by persons skilled in theart.

While two lattice-mismatched (LMM) subcells 22, 24 and one latticeconstant transition layer 20 in any of a variety of ternary and/orquaternary formulations comprising Ga, In, As, and/or P provide wideflexibility in low bandgap designs for efficient absorption andconversion of desired infrared radiation wavelength bands toelectricity, embodiments also extends to three, four, five, or moresubcells and bandgaps with one or more lattice constant transitionlayers, as needed. For example, there is no theoretical limit to thenumber of quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) formulations fordifferent bandgaps between lines 13 and 14 (0.74 eV to 1.35 eV) in FIG.3, which can be lattice-matched on line 12 (5.869 Å). Likewise, there isno theoretical limit to the number of quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) formulations for different bandgaps betweenlines 14 and 17 (0.74 eV to 0.52 eV) in FIG. 3, which can belattice-matched on line 15 (5.968 Å). Further, there is no theoreticallimit to the number of ternary and quaternary Ga, In, As, and/or Pformulations for possible lattice constants between those of InP (5.869Å) and InAs (6.059 Å), lines 12 and 23, respectively, in FIG. 3.

In other words, every ternary Ga_(x)In_(1-x)As or quaternaryGa_(x)In_(1-x)As_(y)P_(1-y) with a bandgap in the range between 0.74 eVand 0.355 eV (lines 14 and 31 in FIG. 3) can be lattice-matched to somehigher bandgap InAs_(y)P_(1-y), which is transparent to at least someinfrared radiation that can be absorbed and converted to electricity bysuch ternary Ga_(x)In_(1-x)As or quaternary Ga_(x)In_(1-x)As_(y)P_(1-y)DH subcells. Further, any InAs_(y)P_(1-y), which is used to make atransition between the lattice constant of such ternary Ga_(x)In_(1-x)Asor quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) to a larger lattice constant,also has a higher bandgap than such Ga_(x)In_(1-x)As orGa_(x)In_(1-x)As_(y)P_(1-y), thus is transparent to at least someinfrared radiation that can be absorbed and converted to electricity bysuch ternary Ga_(x)In_(1-x)As or quaternary Ga_(x)In_(1-x)As_(y)P_(1-y)DH subcells. One or more embodiments described herein utilize thesecharacteristics of Ga_(x)In_(1-x)As_(y)P_(1-y) (0≦x≦1, 0≦y≦1) for thedesign, formulation, and fabrication of low bandgap (less than 1.35 eV,and may be less than 0.74 eV), monolithic, multi-bandgap, photovoltaicconverters, as described above, and as will be further described below.

Embodiments describer by this disclosure, as mentioned above, alsoextends to low bandgap, monolithic, multi-bandgap PV converters withmore than one lattice constant transition layer. For example, referringagain to FIG. 6, one or more additional subcells with even lowerbandgap(s) than the 0.58 eV bandgap of the second subcell 24 can begrown on top of subcell 24. Such an example PV converter 150 with threesubcells 22, 24, 72 is illustrated diagrammatically in FIG. 8. Thisexample three-bandgap PV converter 150 is illustrated for conveniencewith the same substrate 26, first lattice constant transition layer 20,first subcell 22, and second subcell 24 as the two-bandgap embodiment140 of FIG. 6, but it has a second lattice constant transition layer 70positioned between the second subcell 24 and a third subcell 72.

As was explained above in relation to the inverted tandem (two-subcell)PV converter 140 in FIG. 6, the InP substrate 26 and the first latticeconstant transition layer 20 are transparent to infrared radiation oflonger wavelengths than can be absorbed by their respective bandgapcharacteristics. Therefore, in the examples of FIGS. 6 and 8, the lowestbandgap of the InAs_(y)P_(1-y) lattice constant transition layer 20 is0.90 eV (see line 62 in FIG. 7), which, of course, is also lower thanthe 1.35 eV bandgap of the InP substrate. Therefore, infrared radiationof wavelengths longer than 1.38 μm pass through both the InP substrate26 and the InAs_(y)P_(1-y) lattice constant transition layer 20. Thefirst subcell 22, with its 0.70 eV bandgap, absorbs and convertsinfrared radiation R wavelengths of 1.77 μm and shorter to electricity,and it transmits infrared radiation R wavelengths longer than 1.77 μm tothe second subcell 24. The 0.58 eV bandgap of the Ga_(x)In_(1-x)Assecond subcell 24 enables it to absorb and convert infrared radiationwavelengths of 2.14 μm and shorter to electricity, while infraredradiation R wavelengths greater than 2.14 μm pass through the secondsubcell 24.

The energy in the infrared radiation R wavelengths longer than 2.14 μm,which are not absorbed in the second subcell 24 would be wasted in thePV converter 140 embodiment of FIG. 6, but adding one or more additionalsubcells, such as subcell 72 in the three-bandgap PV converter 150 inFIG. 8, can capture and convert significant amounts of that energy toelectricity. However, as shown by line 60 in FIG. 7, there is no ternaryGa_(x)In_(1-x)As or quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) with abandgap below 0.58 eV (line 66) that has the same lattice constant (5.94Å) as the first and second subcells 22, 24 in the example. Therefore, asecond lattice constant transition layer 70 comprising InAs_(y)P_(1-y)with gradually increasing proportions of arsenic (As) is positionedbetween the second subcell 24 and the third subcell 72. The initialInAs_(y)P_(1-y) in the second lattice constant transition layer 70 isformulated to have a bandgap of 0.90 eV, so that it has the same latticeconstant (5.94 Å) as the second subcell 24. Then, the subsequentInAs_(y)P_(1-y) grown for the second lattice constant transition layer70 decreases in bandgap in incremental steps or gradually toward thesame bandgap 66 as the second subcell 24, which is 0.58 eV in theexample described above. At that bandgap level, the InAs_(y)P_(1-y) isstill transparent to all of the infrared radiation R wavelengths thatpass through the second subcell 24. Therefore, the InAs_(y)P_(1-y)second lattice constant transition layer 70 does not absorb or interferewith the infrared radiation R that has to reach the third subcell 72,yet it provides a transition from the lattice constant of the secondsubcell 24 (line 60 in FIG. 7) to a new, larger lattice constant (line74 in FIG. 7) for the third subcell 72. In this example, the new latticeconstant of 6.02 Å will match a ternary Ga_(x)In_(1-x)As with a bandgapof 0.45 eV or quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) with a bandgapanywhere along line 94 between 0.58 eV and 0.45 eV, as shown in FIG. 7.Therefore, if the third subcell 74 in FIG. 8 comprises, for example,Ga_(x)In_(1-x)As formulated to have a bandgap of 0.45 eV, it will absorband convert infrared radiation in wavelengths of 2.76 μm and shorter toelectricity.

Again, as explained above for the first lattice constant transitionlayer 20, the gradual change of lattice constant in the second latticeconstant transition layer 70 can be graded gradually or in discretestepped increments. Also, while not shown in detail in FIG. 8, theGa_(x)In_(1-x)As n/p junction in the third subcell 72 may be clad frontand back with cladding layers of InAs_(y)P_(1-y), which have the samelattice constant as the Ga_(x)In_(1-x)As of the third subcell 74, toform the third subcell 72 as a lattice-matched (LM),double-heterostructure (DH) subcell. Other auxiliary layers mentionedabove can also be provided.

All of the PV converter embodiments 10, 110, 140, 150 described abovehave been monofacial, i.e., grown on only one face of the substrate. Asignificant feature of one or more embodiment is that they can also beimplemented in bifacial or buried substrate structures, as illustrateddiagrammatically by the example low bandgap, monolithic, multi-bandgap,PV converter 80 in FIG. 9. Essentially, in the PV converter 80 of FIG.9, a lattice-matched (LM) first subcell 82 is grown epitaxially on afront surface 83 of a InP substrate 84, and a lattice-mismatched (LMM)second subcell 86 with an intervening lattice constant transition layer90 is grown epitaxially on a back surface 85 of the substrate 84. Anantireflective coating (ARC) 88 on the front surface 81 of the firstsubcell 82 and a back surface reflector (BSR) 89 on the back surface 87of the second subcell 86 are shown, but they can be other opticalcontrol layer materials, as described above for PV converter 10. Again,other auxiliary features, layers, and components that may be used toimplement an actual device, such as buffers, contacts, deep acceptordoping of the InP substrate, tunnel junctions or isolation layers, andthe like are not shown separately in FIG. 9 in order to avoidunnecessary clutter and repetition, but persons skilled in the art canuse the information herein to understand, design, and fabricate suchcomponents in PV converter devices according to embodiment describedherein. Cladding layers (not shown separately in FIG. 9) can be used aspart of the subcells 82, 86 for lattice-matched (LM),double-heterostructure (DH) implementations of the subcells, asdescribed above in relation to the PV converter 10. Also, while only onelattice constant transition layer 90 and two subcells 82, 86 withspecific example bandgaps and lattice constants are illustrated in theexample PV converter 80 in FIG. 9, other numbers of subcells, latticeconstant transition layers, bandgaps, and/or lattice constants can alsobe used, as explained above.

In the example bifacial PV converter 80 in FIG. 9, the first subcell 82is lattice-matched to the InP substrate 84 (line 12 in FIG. 10). In thisexample, subcell 82 is formulated to have the lowest possible bandgapthat can be lattice-matched to the InP substrate 84, which is theternary Ga_(0.47)In_(0.53)As with a bandgap of 0.74 eV (line 14 in FIG.8), although many other formulations could be illustrated, as explainedabove. If it is desired to use a lattice-matched, double-heterostructurefor the first subcell 82, the Ga_(0.47)In_(0.53)As n/p junction materialcan be clad on both sides with lattice-matched, epitaxially grown, InPcladding layers, as described above.

In this example, any incident radiation R of wavelengths shorter than1.67 μm will be absorbed by the 0.74 eV bandgap Ga_(0.47)In_(0.53)As inthe first subcell 82, and longer wavelength infrared radiation R willpass through the first subcell 82. The InP substrate 84, which has amuch higher bandgap of 1.35 eV (line 13 in FIG. 10) is also transparentto any of such longer wavelength infrared radiation that passes throughthe first subcell 82. Therefore, in order for the second subcell 86 toabsorb and convert any of such longer wavelength infrared radiation R toelectricity, it has to have a bandgap E_(g2) that is less than thebandgap E_(g1) of the first subcell 82, i.e., less than 0.74 eV in thisexample. There are many considerations for selecting the lower bandgapfor the second subcell 86, such as targeting the concentrations of theinfrared radiation in various wavelength or frequency bands, conversionefficiencies, and any additional subcells (not shown in FIG. 9).However, any bandgap less than 0.74 eV requires a Ga_(x)In_(1-y)As orGa_(x)In_(1-x)As_(y)P_(1-y) that has a larger lattice constant than theInP substrate 84, so it would not be possible for it to belattice-matched to the InP substrate 84. Therefore, in such anembodiment, a lattice constant transition layer 90 may be needed, andthe second subcell 86 may have a lattice constant that allows theInAs_(y)P_(1-y) lattice constant transition layer 90 to be transparentto the longer infrared radiation wavelengths, which are not absorbed by,and pass through, the first subcell 82. In this example, a bandgapE_(g2) for the second subcell 86 is selected to be 0.55 eV, which can beprovided with ternary Ga_(x)In_(1-x)As having a lattice constant of5.972 Å, as illustrated by lines 92 and 93 in FIG. 10. However, aquaternary Ga_(x)In_(1-x)As_(y)P_(1-y) with a slightly larger latticeconstant could also be used for the same bandgap and still be able toaccommodate a transparent lattice constant transition layer 90.

As shown by lines 12, 93 in FIG. 10, the ternary Ga_(x)In_(1-x)As withits lattice constant of 5.972 Å requires the lattice constant transitionlayer 90 to make the transition between the lattice constant of the InPsubstrate (line 12) to the 5.972 Å lattice constant (line 93). As alsoillustrated in FIG. 10, increasing the proportion of As in anInAs_(y)P_(1-y) lattice constant transition layer 90 reaches this 5.972Å constant without its bandgap ever decreasing below about 0.82 eV (line94), which is still higher than the 0.74 eV bandgap (line 14) of thefirst subcell 82. Therefore, any infrared radiation R that is notabsorbed by, thus passes through, the first subcell 82 will also not beabsorbed by the InAs_(y)P_(1-y) in the lattice constant transition layer90. Consequently, such infrared radiation will reach the second subcell86, where at least some of it can be absorbed and converted toelectricity.

If the substrate 84 is doped with a deep acceptor element, such as Fe orCr, to be an insulator or semi-insulator, as explained above, then thefirst subcell 82 and the second subcell 86 are electrically isolatedfrom each other. Therefore, electricity has to be extractedindependently from each subcell 82, 86, as described above for theelectrically isolated subcells 22, 24 of the PV converter device 110 inFIG. 4. This feature has advantages, such as in voltage-matching ofmultiple, series and/or parallel interconnected PV converter subcellcircuits, especially in monolithic, integrated module (MIM) devices, aswill be described in more detail below. In situations where thesubstrate 86 cannot be made as an insulator or semi-insulator, aseparate isolation layer (not shown in FIG. 9) can be positionedanyplace between the two subcells 82, 86. For example, an isolationlayer can be grown on either the front surface 83 or back surface 85 ofthe substrate 84 or between the lattice constant transition layer 90 andthe second subcell 86. Such an isolation layer can be made as desiredabove in relation to the isolation layer 39 in FIG. 4, i.e., alattice-matched material that is transparent to wavelengths of radiationnot absorbed by the first subcell 82 and doped to make the materialhighly-resistive or to create a diode barrier to the flow of electriccurrent. Of course, there could also be applications that involve aseries connection of subcell 82 on the front side of the substrate 84with the subcell 86 on the back side of the substrate 84, in which casethe substrate 84 should be doped to conduct current, and appropriatetunnel junction layers may be added to allow current flow as explainedabove in relation to the PV converter device 10 of FIG. 2.

As explained above, any of a wide range of ternary or quaternaryGaInAs(P) alloys with any combinations of bandgaps and lattice constantscan be used in subcells of tandem (more than one subcell) stacks oflow-bandgap, monolithic, multi-bandgap, optoelectronic devices. Anotherexample of such combinations is illustrated in the alternate examplebifacial PV converter device 160 in FIG. 11, where two latticemismatched (LMM) subcells 162, 164 are grown on the front side 165 of anInP substrate 166 and another, even lower bandgap, subcell 168 is grownon the back side 167 of the substrate 166. For purposes of thisillustration, but not for limitation, the first subcell 162 is shown asLM, DH, quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) with a bandgap of 1.1 eV,but lattice-matched to a LM, DH, ternary Ga_(x)In_(1-x)As second subcell164 instead of to the InP substrate 166, as shown by lines 170, 172, 174in FIG. 10. Therefore, a lattice constant transition layer 176, such asgraded InAs_(y)P_(1-y) is needed between the InP substrate 166 and thesecond subcell 164, as shown in FIG. 11, to make the transition betweenthe 5.869 Å lattice constant of the InP substrate 166 (line 12 in FIG.10) and the 5.905 Å lattice constant of the ternary Ga_(x)In_(1-x)As ofthe second subcell 164 (line 174 in FIG. 10). Either an isolation layer178, as shown in FIG. 11, or a tunnel junction, can be positionedbetween the first subcell 162 and the second subcell 164, depending onwhether it is desired to connect the subcells 162, 164 independently orin series, as explained above.

The third subcell 168 in the PV converter 160 in FIG. 11 has an evenlower bandgap, for example 0.55 eV, to absorb and convert longerwavelength radiation transmitted through the first and second subcells162, 164 to electricity, according to the principles explained above.Such a third subcell 168 can be, for example, a LM, DH, ternaryGa_(x)In_(1-x)As with a 0.55 eV bandgap, which is not lattice-matched tothe InP substrate 166, as shown by lines 12, 93 in FIG. 10. Therefore,another lattice constant transition layer 180, such as gradedInAs_(y)P_(1-y) is needed between the InP substrate 166 and the thirdsubcell 168 to make the transition between the 5.869 Å lattice constantof InP (line 12 in FIG. 10) and the 5.952 Å lattice constant of theternary Ga_(x)In_(1-x)As (line 93 in FIG. 10) of the third subcell 168.

Again, if the InP substrate 166 is deep acceptor doped to be aninsulator or semi-insulator, the third subcell 168 will be electricallyisolated from the first and second subcells 162, 164 and can beconnected independently to other PV converters or subcells, such as in aMIM structure (described below). Otherwise, a separate isolation layer(not shown in FIG. 11) may be needed somewhere between the substrate 166and the third subcell 168, as explained above. Again, contacts,conductive layers, buffers, optical control layers, and the like, arenot shown in FIG. 11, but can be provided as explained above for otherembodiments.

Another interesting variation of the bifacial embodiment PV converter 80in FIG. 9 is the use of epitaxially grown InP for the higher bandgapfirst subcell 82 instead of a ternary or quaternary GaInAs(P). Thisvariation, with an appropriate lower bandgap (lower than the 1.35 eVbandgap of InP) second subcell 86, can operate as a highly efficient,stand-alone, tandem solar cell. This bifacial or buried substrateconfiguration of the PV converter 80 is particularly advantageous foruse as a solar cell, because the buried InP substrate 84 is not in aposition to block or absorb shorter wavelength solar radiation before itreaches the first subcell 82.

An illustration of this principle in a slightly more complex bifacial,monolithic, multi-bandgap, solar photovoltaic (SPV) converter device190, multiples of which can also be incorporated into a MIM structure,is shown in FIG. 12. In this example SPV device 190, there are threelattice-matched subcells 192, 194, 196 grown on the front side 197 of aInP substrate 198 and two, lower bandgap, lattice-mismatched (LMM)subcells 200, 202 grown on the back side 199 of the substrate 198.Again, isolation and/or tunnel junction layers 204, 206 can be includedbetween front-side subcells 192, 194 and/or between subcells 194, 196,respectively, for either independent electrical connection or serieselectrical connection, respectively, within the SPV device 190, asexplained above. Similarly, either an isolation layer 208 or a tunneljunction can be provided between the back-side subcells 200, 202,depending on whether it is desired to electrically connect themindependently or in series within the SPV device 190.

In the example SPV device 190, the first subcell 192 is shown as a LM,DH, InP subcell with a bandgap of 1.35 eV, while the bandgaps of thesecond and third subcells 194, 196 have lower bandgaps, e.g., 1.0 eV,LM, DH, quaternary Ga_(x)In_(1-x)As_(y)P_(1-y) for the second subcell194 and 0.74 eV, LM, DH, ternary Ga_(0.47)In_(0.53)As for the thirdsubcell 196, which is the lowest bandgap GaInAs that can belattice-matched to the InP substrate 198 (see lines 13, 14, 210 in FIG.13). Since all the front-side subcells 192, 194, 196 are lattice-matched(line 12 in FIG. 13) to the InP substrate 198, no lattice constanttransition layer is needed on the front side of the substrate 198.

Again, the variations of conductive or highly-resistive substrate 198,isolation and/or tunnel junction layers, electrical contacts, bufferlayers, optical control layers, more or fewer subcells, differentbandgaps, and the like, as described above for other embodiments, arealso applicable to the SPV device 190 described above.

Also, AlInAs in slightly higher bandgaps than the 1.35 eV bandgap of theInP can also be lattice-matched to InP, so a first subcell of suchAlInAs lattice-matched to the InP substrate could also be used as partof a bifacial, monolithic, multi-bandgap, PV converter. Of course, Gacould be added to produce AlGaInAs, if a slightly lower bandgap thanAlInAs may be desired for either the first subcell or a subsequentsubcell of a bifacial PV converter.

The PV converters described above can be used alone or in combinationswith myriad other devices. For example, any of the PV converters,especially the SPV device 190, but also, PV converters 10, 110, 112,140, 150, 80, 160, can be used for the bottom cell device in amechanical stack of higher bandgap (higher than the 1.35 eV bandgap ofInP) PV converters, such as GaAs based PV converters, in solar cell andother applications. Such other, higher bandgap, PV converters (notshown) can selectively absorb and convert shorter wavelength solarenergy to electricity, while the lower bandgap PV converters, e.g., PVconverters 10, 110, 112, 150, 80, 160, 190, absorb and convert longerwavelength solar radiation to electricity.

As mentioned above, one or more of the bifacial, monolithic,multi-bandgap, optoelectronic devices described herein, for example, thebifacial PV converters 80, 160, 190 shown in FIGS. 9, 11, and 12 anddescribed above, as well as myriad variations of such bifacialconfigurations, are particularly adaptable to use in monolithic,integrated modules (MIMs). An example MIM PV converter device 230 with aplurality of bifacial, monolithic, multi-bandgap, photovoltaicconverters 190 of FIG. 12 grown on a single substrate 198 is shown inFIG. 14. Essentially, all of the PV converter subcell stacks 190 aregrown in unison on the common substrate 198, and then they are separatedinto a plurality of individual subcell stacks 190′ by etching away orotherwise removing material to form isolation trenches 232 between thefront-side subcell stacks 190′ and to form isolation trenches 234between the back-side subcell stacks 190″. Then, various conductors 236,238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, and othersare added in various electrical connection patterns to interconnect thesubcells together, as will be described in more detail below. The spacesbetween the conductors are filled with insulator material, such assilicon nitride or any of a variety of other suitable insulatormaterials.

In the bifacial MIM PV converter device 230 illustrated in FIG. 14,there are twice as many, albeit smaller, back-side subcell stacks 190″as front-face subcell stacks 190′. Further, there can be any desiredratio of back-side subcell stacks 190″ to front-side subcell stacks190′. The ratio of two back-side subcell stacks 190″ to one front-sidesubcell stack 190′ shown in FIG. 14 is only an example.

One advantage of being able to have different numbers of subcell stacks190′, 190″ on front and back of the substrate 198 is more flexibility todesign voltage-matched subcell circuits. A schematic diagram of anequivalent electrical circuit corresponding to the examplevoltage-matched subcell circuits 262, 264, 266, 270, 272 of the MIM 230of FIG. 14 is shown in FIG. 15. according to fundamental electricalprinciples, a circuit comprising a plurality of subcells connected inparallel will have an output voltage equal to the subcell outputvoltage, but current from the parallel connected subcells add.Conversely, a circuit comprising a plurality of subcells connected inseries will have a current output equal to the subcell output current,but the output voltages of the series connected subcells add. Therefore,connecting a plurality of higher voltage subcells together in parallelcan build current as output voltage remains constant, while connecting aplurality of lower voltage subcells together in series can boost thevoltage output of the subcell circuit to the level of the higher voltagesubcell circuit. Also, higher bandgap subcells produce higher voltagethan lower bandgap subcells. Therefore, if, for example, the voltageoutput of each of the lower bandgap subcell stacks 190″ in FIG. 14 ishalf as much as the voltage output of each of the higher bandgap subcellstacks 190′, and if all the higher bandgap subcell stacks 190′ areconnected in series with each other while all the lower bandgap subcellstacks 190″ are connected in series with each other, then the totalvoltage output of the back-side subcell stacks 190″ circuits 270, 272would equal the total voltage output of the front-side subcell stacks190′ circuits 262, 264, 266, because there are twice as many low voltagesubcell stacks 190″ as there are higher voltage subcell stacks 190′.

However, the front-side subcells 192, 194, 196 of the front-side stacks190′ can be connected in myriad combinations of series and/or parallelelectrical connections, as illustrated in FIGS. 14 and 15 to createvoltage-matched subcell circuits 262, 264, 266. The same goes for theback-side subcells 200, 202 of the back-side stacks 190″ to createvoltage-matched subcell circuits 270, 272. Such electrical connectionoptions are facilitated by the isolation layers 204, 206, 208 andhighly-resistive substrate 198, as described above. Additional optionscan be provided by tunnel junctions instead of isolation layers or evenmaking the substrate 198 conductive rather than resistive forintra-subcell stack series connections, as explained above.

To illustrate several series and parallel connection options, thebifacial MIM PV converter device 230 in FIGS. 14 and 16, is shown, forexample, with all of its highest bandgap, thus highest voltage, subcells192 connected together in parallel to form the subcell circuit 262. Thenext highest bandgap, thus next highest voltage, subcells 194 areconnected together in a combination of parallel 244, 246 and series 247connections to form a subcell circuit 264 that is voltage-matched to thesubcell circuit 262. The subcells 196, which are the lowest bandgap,thus lowest voltage, of the subcells on the front side of the substrate198, are shown in this example illustration of FIGS. 14 and 15, as allbeing connected in series in subcell circuit 266 by conductors 242 toadd their voltages in order to match the output voltage of subcellcircuit 266 to the output voltages of the subcell circuits 262, 264.These parallel and series-connected subcell circuits 262, 264, 266 areconnected in parallel to each other at conductors 236, 238, 240 and at236′, 238′, 240′ to add their respective current outputs.

The back-side subcells 200, 202 are even lower voltage than thefront-side subcells 196, but there are more of them than the front-sidesubcells 192, 194, 196, so the back-side voltage can be matched to thefront-side voltage. In the example of FIGS. 14 and 15, the lowestvoltage subcells 202 are connected together in series in the subcellcircuit 272 by conductors 260 to add their voltages in order to matchthe output voltage of subcell circuit 272 to the output voltage of theparallel connected 256, 258, higher voltage subcells 200 in the subcellcircuit 270. Then, the subcell circuit 272 is connected in parallel tothe subcell circuit 270 by conductors 252, 254 and 252′, 254′ to addtheir respective current outputs.

Finally, the front-side subcell circuits 262, 264, 266 are connected inparallel to the back-side subcell circuits 270, 272 at terminal contacts258, 258′ to add their respective current outputs. Therefore, thebifacial MIM PV converter 230 can be connected electrically to otherdevices or loads via the two terminal contacts 256, 258, which may be adesirable feature. Other MIM structures, circuit connections, andadvantages can be made according to these principles within this scopeof embodiments. For example, but not for limitation, the monofacial,monolithic, LM, DH, multi-bandgap, PV converters described above canalso be incorporated into MIM structures (not shown), although thebifacial embodiments described above have the advantage of using thesubstrate 198 as a built-in isolation structure between subcells on thefront side and subcells on the back side, as explained above.

Any of the PV converter embodiments 10, 110, 150, described above andshown in FIGS. 1, 2, 4, and 6 can be modified to provide an ultra-thin,monolithic, multi-bandgap, PV converter by fabricating it in such a wayas to enable removal of the InP substrate 26. For example, as shown inFIG. 16, a monolithic, multi-bandgap (tandem), PV converter 100 isfabricated much the same as the PV converter 10 in FIG. 2 on an InPsubstrate 26, except that a stop-etch layer 98 is added between thebuffer layer 32 and the front cladding layer 40 of the first subcell 22.The stop-etch layer 98 can be, for example, n-Ga_(0.47)In_(0.53)As withthe same lattice constant as the InP substrate 26, so that thesubsequent layers of the first and second subcells 22, 24, tunneljunction 44, 46, (or isolation layer for independently connectedsubcells) and lattice constant transition layer 20 can be grownepitaxially, as described above.

The purpose of the stop-etch layer 98 is to enable the InP substrate 26and buffer layer 32 to be removed by etching or other selective chemicalremoval to create an ultra-thin, monolithic, multi-bandgap (tandem) PVconverter 100 without etching or damaging any of the first subcell 22.After the several layers of the structure in FIG. 16 are grownepitaxially on the InP substrate 26, the structure 100 is top-mounted onanother object 102, such as a solar panel, heat sink, printed circuitboard, or other useful platform. Then the substrate 26 and buffer layer32 are removed by etching or other selective chemical removal, leavingthe ultra-thin, monolithic, multi-bandgap, PV converter 100 mounted onthe object 102, as shown in FIG. 17. The stop-etch layer 98 can be anelectrically conductive material, so it can also serve as a contactlayer. If desired, part of such a conductive stop-etch layer 98 can beremoved by etching or other selective chemical removal with a differentchemical in which it is soluble to leave a grid pattern, which would beuseful if the material of layer 98 is not transparent to the incidentradiation R.

Mounting the PV converter 100 on the object 102 can be accomplished witha suitable adhesive or by any other suitable mounting mechanism. Ananti-reflective coating 97 can be added to reduce reflection of incidentradiation, or layer 97 can be any other optical control material forpurposes described above for the PV converter 10.

This ultra-thin, monolithic, multi-bandgap, PV converter 100 enablesthis device to be used as a solar cell, because elimination of the InPsubstrate 26 allows all of the incident solar radiation SR to reach thesubcells 22, 24, which can convert it to electricity. Otherwise, the InPsubstrate 26, which has a bandgap of 1.35 eV, would absorb large amountsof solar radiation SR in wavelengths shorter than 0.93 μm, before suchsolar radiation SR could reach the first subcell 22. There is no n/pjunction in the substrate 22, and it cannot convert radiant energy toelectricity, so any solar energy absorbed by the substrate 26 would bethermalized and wasted as heat.

Even without the InP substrate, however, there could be significantproduction of heat in the PV converter 100, when it is used as a solarcell, because there is a substantial amount of energy in higherfrequencies (shorter wavelengths) of the solar spectrum, wherewavelengths are substantially shorter than the longest wavelength thatcan be absorbed by the first subcell 22. Therefore, there is significantthermalization of excess energy that is not needed for carriers totranscend the bandgap E_(g1) of the first subcell 22, thus a significantproduction of heat that should be dissipated from the PV converter 100.However, the PV converter 100 is ultra-thin and has no thick substrate,so heat can flow through the PV converter 100 is substantiallyone-dimensional, and it can flow quickly and easily to the back surface104. If the object 102 on which the PV converter 100 is mounted is agood heat sink, i.e., good thermal conductivity and sufficient massand/or surface area to conduct heat away from the PV converter 100, thecombination provides very good thermal management and minimizes heatbuild-up in the PV converter 100.

The ultra-thin, monolithic, multi-bandgap, PV converter 100 can also begrown in a polycrystalline form on less expensive substrates, such asgraphite, which is amorphous and does not impose a lattice constant onthe first subcell 22, or in single-crystal form on compliant substrateor bonded substrate systems, which provide a lattice constant match toaccommodate epitaxial growth. A typical compliant substrate may be made,for example, with an inexpensive substrate material, such as silicon,and with an amorphous oxide of the substrate material followed by alayer of perovskite oxide. Therefore, a first subcell 22 of InP, GaInAs,or GaInAsP will grow with its natural lattice constant. Such firstsubcell 22 can then be followed by a InAs_(y)P_(1-y) lattice constanttransition layer 20 and another, lower bandgap, second subcell 24, asdescribed above. Then, the resulting ultra-thin PV converter 100 ismounted on another object 102 and the compliant substrate is removed.

Compliant substrates can also be used on any of the monofacial PVconverter embodiments 10, 110, 112, 140, 150 described above, and theycan possibly be used for the bifacial embodiments 80, 160, 190, 230.Possible uses of compliant substrates in one or more embodimentsdescribed herein depend on the transparency and other properties of thecompliant substrate materials and systems being considered and/orapplied.

While the description of this provided in this disclosure has focusedprimarily on photovoltaic converters, persons skilled in the art knowthat other optoelectronic devices, such as photodetectors andlight-emitting diodes (LEDs) are very similar in structure, physics, andmaterials to PV converters with some minor variations in doping and thelike. For example, photodetectors can be the same materials andstructures as the PV converters described above, but perhaps morelightly-doped for sensitivity rather than power production. On the otherhand LED's can also be much the same structures and materials, butperhaps more heavily-doped to shorten recombination time, thus radiativelifetime to produce light instead of power. Therefore, embodiments alsoapply to photodetectors and LEDs with structures, apparatus,compositions of matter, articles of manufacture, and improvements asdescribed above for PV converters.

Since these and numerous other modifications and combinations of theabove-described method and embodiments will readily occur to thoseskilled in the art, it is not desired to limit embodiments to the exactconstruction and process shown and described above. For example,accordingly, resort may be made to all suitable modifications andequivalents that fall within the scope of the disclosure as defined bythe claims which follow. The words “comprise,” “comprises,”“comprising,” “include,” “including,” and “includes” when used in thisspecification and in the following claims are intended to specify thepresence of stated features or steps, but they do not preclude thepresence or addition of one or more other features, steps, or groupsthereof. Also, GaInAs(P) is used as a shorthand, generic term thatincludes any ternary Ga_(x)In_(1-x)As and/or quaternaryGa_(x)In_(1-x)As_(y)P_(1-y), and similar notation conventions apply toAlGaInAs(P).

The invention claimed is:
 1. A monolithic, integrated, module (MIM),comprising: a plurality of monolithic, multi-bandgap, photovoltaicconverters, each of which comprises: (i) a first subcell with a firstbandgap and a first lattice constant; (ii) a second subcell with asecond bandgap and a second lattice constant, wherein the second bandgapis less than the first bandgap and the second lattice constant isgreater than the first lattice constant; and (iii) a lattice constanttransition material positioned between the first subcell and the secondsubcell, said lattice constant transition material having a bandgap atleast as large as the first bandgap and a lattice constant that changesfrom the first lattice constant to the second lattice constant; and acommon substrate with a substrate bandgap and a substrate latticeconstant, said common substrate being positioned between the latticeconstant transition material and the second subcell of each of themonolithic, multi-bandgap, photovoltaic converters, wherein thesubstrate bandgap is at least as large as the first bandgap and thesubstrate lattice constant is equal to the second lattice constant. 2.The monolithic, integrated, module (MIM) of claim 1, wherein the latticeconstant transition materials and the first subcells are grownepitaxially on a front side of the substrate, and wherein the secondsubcells are grown epitaxially on a back side of the substrate.
 3. Themonolithic, integrated, module (MIM) of claim 1, wherein each of thefirst subcells comprises GaInAs(P), each of the second subcellscomprises GaInAs(P), each of the lattice constant transition materialscomprises InAs_(y)P_(1-y), and the substrate comprises InP.
 4. Themonolithic, integrated, module (MIM) of claim 1, further comprising atunnel junction positioned between the first subcell and the secondsubcell of each of the monolithic, multi-bandgaps, photovoltaicconverters.
 5. The monolithic, integrated, module (MIM) of claim 4,wherein the tunnel junction is positioned between the substrate and thesecond subcell.
 6. The monolithic, integrated, module (MIM) of claim 1,further comprising an isolation layer positioned between the firstsubcell and the second subcell of each of the monolithic, multi-bandgap,photovoltaic converters.
 7. The monolithic, integrated, module (MIM) ofclaim 6, wherein a subcell circuit comprising the first subcells isvoltage-matched to a subcell circuit comprising the second subcells. 8.A monolithic, multi-bandgap, photovoltaic converter comprising: a firstsubcell with a first bandgap and a first lattice constant; a secondsubcell with a second bandgap and a second lattice constant, wherein thesecond bandgap is less than the first bandgap and the second latticeconstant is greater than the first lattice constant; a lattice constanttransition material positioned between the first subcell and the secondsubcell, said lattice constant transition material having a bandgap atleast as large as the first bandgap and a lattice constant that changesfrom the first lattice constant to the second lattice constant; and asubstrate with a substrate bandgap and a substrate lattice constant,said substrate being positioned between the lattice constant transitionmaterial and the second subcell, wherein the substrate bandgap is atleast as large as the first bandgap and the substrate lattice constantis equal to the second lattice constant.
 9. The monolithic,multi-bandgap, photovoltaic converter of claim 8, wherein the latticeconstant transition material and the first subcell are grown epitaxiallyon a front side of the substrate, and wherein the second subcell isgrown epitaxially on a back side of the substrate.
 10. The monolithic,multi-bandgap, photovoltaic converter of claim 8, wherein the firstsubcell comprises GaInAs(P), the second subcell comprises GaInAs(P), thelattice constant transition material comprises InAs_(y)P_(1-y), and thesubstrate comprises InP.
 11. The monolithic, multi-bandgap, photovoltaicconverter of claim 8, further comprising a tunnel junction positionedbetween the first subcell and the second subcell.
 12. The monolithic,multi-bandgap, photovoltaic converter of claim 11, wherein the tunneljunction is positioned between the substrate and the second subcell. 13.The monolithic, multi-bandgap, photovoltaic converter of claim 8,further comprising an isolation layer positioned between the firstsubcell and the second subcell.